Apache 2 is not a built-in application. Instead, it is available as a 3rd party package, installable via pkg_add(1). The name of the package is apache-httpd. This was used to differentiate it, historically, from OpenBSD's fork of Apache 1.3 that used to be a built-in application. OpenBSD's Apache 1.3 is still available, as a package: apache-httpd-openbsd.

If you're unfamiliar with pkg_add(1) and OpenBSD's other third party package management tools, please refer to FAQ 15.

I know perfectly how to add a package... Apache 2 is properly installed on the system. It just don't start nor with sudo apachectl2 start neither with sudo httpd2 -k start.
I get absolutely no error message, the syntax of the conf file is ok. No httpd2 process runs after those commands.

-e Sets the LogLevel to level during server startup. This is useful
for temporarily increasing the verbosity of the error messages
to find problems during startup.
-X Run httpd2 in debug mode. Only one worker will be started and
the server will not detach from the console.